1. Technical Field of the Invention
The present invention relates generally to fluid flow control and, more particularly, to a high flow capacity positioner for regulating fluid flow within a fluid circuit.
2. Description of the Related Art
A control valve regulates a flowing fluid, such as gas, steam, water, or chemical compounds. An actuator may be used to regulate the flow of fluid within a control valve. The actuator provides motive power to open or close the valve and therefore regulate the fluid flow within the valve.
A valve positioner is a device mounted on the actuator that may implement a control strategy determined by an output of a controller in electrical communication with the valve positioner. The controller provides a variable current signal to the valve positioner. The variable current signal is proportional to the state of the valve positioner. For example, a valve positioner may fully open a valve in response to a 4 milliamp (mA) current signal and fully close the valve in response to a 20 mA current signal. The valve positioner compares the current signal to the actuator's position to provide the motive force necessary to move the actuator accordingly. If the current signal differs from the actuator's position, the valve positioner moves the actuator until the correct position is reached. Valve positioners are well known in the art. Two types of well-known positioners include a single-acting pneumatic positioner which sends air and exhausts air from one side of the actuator that is opposed by a range spring, and a double-acting pneumatic positioner which sends air and exhaust air from both sides of the actuator. Valve positioners have been greatly improved upon through the use of digital devices that use microprocessors to position the actuator, monitor key variables, and implement control algorithms and record data.
The actuator converts energy in the form of compressed air into motion including linear or rotary motion. The actuator is configured to receive a large volume flow of air in order to be displaced to a desired position for regulating fluid flow. There are well known advantages for using compressed air rather than hydraulics to provide motive force to the actuator. These advantages include exhausting air rather than oil into the atmosphere. Using compressed air is also better suited for absorbing excessive force. Furthermore, stored air may be used when power to the valve positioner is lost. Additionally, minimal maintenance is required for actuators that are dependent upon the use of compressed air. Using a valve positioner to control the movement of the actuator by communicating microprocessor-based current is widespread for enhanced accuracy and efficiency. The ability to more accurately and efficiently control the actuator is due in large part to the controller of the valve positioner. As indicated above, the controller is known to receive feedback of the valve travel position and adjust the current that is representative of the desired actuator position for controlling a particular process. Valve positioners are known to convert the current signal outputted by the controller into a pressure signal used to supply the actuator with a quantifiable amount of compressed air. Valve position feedback is critical to the operation of the valve positioner. Without feedback, the control valve may default to its failsafe position or a random position.
The ability of the valve positioner to precisely regulate the flow rate of fluid within a fluid circuit is an important characteristic. The valve positioner is employed to move the actuator which is conventionally comprised of a piston sealed within a cylinder. The valve positioner moves the piston by forcing compressed air into one end of the cylinder while simultaneously withdrawing or exhausting the compressed air out of an opposing end of the cylinder. Valve positioners are most often used in closed-loop systems where the position of the actuator, and velocity and/or pressure of the compressed air flowing within the valve positioner, is continuously monitored with a feedback device which generates system feedback signals. The controller uses the system feedback signals to generate current signals that are received by the valve positioner to minimize the error between a desired position of the piston and an actual position of the piston within the cylinder.
Valve positioners generally incorporate a spool that either rotates or slides axially in a housing to port the compressed air flow to the actuator or direct air flow from the actuator to one or more exhaust ports. However, for valve positioner systems adapted for providing high fluid flow rates, relatively large spools are incorporated into the positioner. Larger spools require relatively large moments of inertia, necessitating the employment of a stepper motor having relatively high torque output. Displacing a large spool using an electric motor means that the device needs a separate power source. Valve positioners that use stepper motors typically position the spool in an open-loop fashion wherein the spool must be initialized. Positioning the spool in an open-loop design is more susceptible to delay than a closed-loop design and is not recommended for applications requiring increased accuracy and efficiency. During initialization, the spool is moved to a starting point or initialization position from where the stepper motor may initiate movement of the spool to a desired position. The controller may command the stepper motor to move the spool so that the controller may track a sequence of current signals from the initialization position and maintain a virtual spool position in its memory. As long as the stepper motor precisely tracks the sequence of driver signals, the error between the desired position and the actual position of the piston of the actuator is minimized. Using a stepper motor to displace the spool has well known disadvantages. The torque of the motor may generate unwanted spool rotation. Displacing the spool using a stepper motor may also contribute to misalignment between a thrust axis and the spool axis having a negative impact on the accuracy of the valve positioner.
Another well-known method used to move the spool within the valve positioner is the use of a pressure responsive diaphragm. The diaphragm is capable of receiving fluid pressure in the form of compressed air to exert a force on the spool, causing the spool to move. The diaphragm is also configured to release air to exert a force in an opposite direction from the direction of spool movement. Therefore, the diaphragm may be used to move the spool bi-directionally relative to a housing in which the spool is disposed. However, diaphragms are not recommended when higher flow capacity is desired. Higher flow capacity requires larger spools, which require more force to move the spool. Under this circumstance, using a pressure responsive diaphragm may not be feasible. This presents the dilemma of whether to use the stepper motor to displace a larger spool for increased flow capacity or to use the pressure responsive diaphragm to displace a smaller sized spool. The desired flow capacity may influence whether the stepper motor or the pressure responsive diaphragm is used.
Valve positioner performance can be rated based upon flow capacity. Flow capacity is measured by the flow coefficient (Cv). Cv is linearly related to the flow capacity of the valve positioner. For example, an increase in the Cv corresponds to an increased flow capacity. The flow coefficient of a device is a relative measure of its efficiency of fluid flow within a fluid circuit. The Cv describes the relationship between the pressure drop across a fluid circuit and the corresponding flow rate. The Cv is the volume (US Gallons) of water at 60 degrees Fahrenheit that will flow per minute through a valve with a pressure drop of 1 pound per square inch (psi) across the valve. For example, Cv=22 means 22 gallons of water at 60 degrees Fahrenheit will flow through a valve with a pressure drop of 1 psi across the valve. The use of the Cv offers a standard method of comparing valve capacities and sizing valves for specific applications that are widely accepted in a particular industry.
Often valve positioners with greater flow capacity are preferred because of their ability to move a greater amount of fluid. However, a valve positioner system with increased flow capacity is more expensive and complex to manufacture. Increased flow capacity is associated with faster spool stroke times. The faster the spool stroke time, the faster the actuator must be driven to a desired position. Additionally, faster spool stroke time is associated with better frequency response. Faster spool stroke time translates into better response to a small current signal change. Alternatively, slower spool stroke time correlates to a reduced flow capacity rating. The longer it takes for the spool to move from one position to another position, the longer it takes to drive the actuator to the desired position. Longer spool stroke time results in reduced frequency response and therefore a reduced ability to respond to smaller changes in the current signal from the controller. Minimizing the range in which the spool may move is one method that may be employed for faster spool stroke time. However, faster spool stroke time may result in decreased accuracy. Thus, for applications where accuracy is of greater importance, it may be desirable to reduce the spool stroke time for better spool positioning accuracy.
Improving a valve positioner system for widespread use requires attention to various factors including, for example, manufacturing costs, power consumption and flow capacity. The power consumption associated with a valve positioner is typically associated with a current loop source. The current loop is a communication interface that uses current instead of voltage for signaling from the controller to the valve positioner. A popular and widely used industry standard includes a 4-20 mA current loop range. Thus, it is important for the valve positioner to function effectively within the 4-20 mA current source.
Increasing the flow capacity of the valve positioner requires a larger spool with faster spool stroking time. The force required to move the larger spool faster is greater. Faster spool stroke time for a particular range of spool movement requires greater thrust, which in turn requires more power. The increase in force may be compensated by increasing the power consumption of the valve positioner. However, maintaining the power consumption contemplated by a 4-20 mA current loop is also an important consideration because of its wide use in industry. If the power consumed requires a current loop source greater than 4-20 mA, the valve positioner may not be accepted for certain applications that rely on the 4-20 mA standards. Therefore, commercial success of the valve positioner system may hinge on remaining within the industry wide standard of 4-20 mA current loops.
Designing a high flow capacity positioner system within a particular cost and power consumption range is limited in some respects. A well-known method used to increase the flow capacity of a valve positioner includes the use of boosters. Boosters amplify the volume of air supplied to the actuator. Although valve positioners fitted with boosters provide more Cv capacity, the disadvantages include a significant decrease in control quality. In this regard, the boosters are mechanical-pneumatic flow amplifiers driven by positioner flow with intrinsic lag time. As a result, a high amplification ratio gives rise to instability, with a low amplification ratio often not meeting dynamic performance requirements. The addition of boosters also includes the addition of piping and fittings resulting in an increase in cost and parts. The extra parts make maintenance more difficult, also there is potential increase in the risk of malfunctions and/or leakages. Furthermore, boosters must be calibrated and adjusted which takes time and money.
Accordingly, there exists a need in the art for a valve positioner which addresses one or more of the above or related deficiencies.